Optical communication systems transmit data using electromagnetic light signals in optical fiber and/or free space (for example, building to building, ground to satellite, satellite to satellite, etc.). The electromagnetic carrier wave is modulated to carry the data. Optical communication in optical fiber typically involves: generating the optical signal, relaying the signal on an optical fiber (including measures to reduce/mitigate attenuation of, interference with and/or distortion of the light signal), processing a received optical signal, and converting the signal into a useful electrical signal. Transmitters can be semiconductor devices such as laser diodes, producing coherent light for transmission. A number of receivers have been developed for processing a transmitted lightwave optical signal to provide processed optical signal input(s) to one or more photodetectors, which convert light into electricity.
A coherent receiver, such as an Integrated Coherent Optical Receiver (ICR), converts a modulated optical signal into four electrical signals corresponding to an “in-phase” (I) and “quadrature” (Q) optical signal components of the two optical polarization states, vertical and horizontal. These components can be processed to recover the optically transmitted data regardless of modulation type. Together these four output electrical signals carry all or nearly all of the information conveyed by the optical signal. The electrical outputs of the ICR provide the I and Q mixer signals for the two polarizations.
Testing an ICR presents a special challenge in that the output stage is a balanced detector pair often followed by a differential amplifier with differential outputs. The fact that there are four differential outputs (I and Q each for X and Y polarizations), compounds the difficulty. A simple coherent receiver is composed of a local-oscillator laser, an optical coupler, and one or more photodetectors that can be in a “balanced” configuration that cancels photocurrents and eliminates DC terms and the related excess intensity noise.
The balanced detection and differential amplification of the ICR ensure that any signal put into only the signal port or only the Local Oscillator (LO) port of the ICR will be rejected unless it is possible to block one of the photodiodes to break the balanced detection. Although early versions of ICRs allowed physical access to interrupt a light signal and thereby break the balanced detection, this is not possible on modern integrated components, which are instead typically intrinsically sealed. Getting any meaningful signal out of the ICR therefore requires both a signal and a local oscillator input. A problem is that the optical LO input must be phase coherent with the test signal. The precise optical phase of the local oscillator signal will affect whether either one or both of the output diodes are illuminated with the test signal.
Since getting any meaningful signal out of the ICR requires both a signal and a local oscillator input, both the frequency and phase relationship between the signal and local oscillator are important. While it is routine to simply connect two single-frequency lasers, one to the Signal and one to the LO port to get a beat-frequency output at the frequency difference between the lasers, this method is good only for determining the magnitude of the frequency response. The phase response of a coherent receiver is also important. In addition to maintaining 90-degree relative phase between I and Q outputs, the receiver also must have low group-delay and skew variation over modulation frequency as well as good Common Mode Rejection Ratio (CMRR) vs. frequency.
Measuring phase response vs. frequency requires a stable phase reference unless both low and high frequency components are supplied simultaneously with a known phase relationship. For example, a pulsed laser has both low and high frequency harmonics simultaneously. However, the pulsed laser still requires some sort of LO input which creates a very similar difficulty to the frequency domain approach.
Embodiments of the invention address these and other limitations of the prior art.